[BC] Xmitters at aol.com

Wed Apr 11 07:07:20 CDT 2007

No! No! No!

The transformer does __NOT__ represent an impedance in itself. The transformer is specified to work in a circuit with certain impedances. To do this, it was designed so that its own reactances and resistance (complex impedances) are inconsequential for the range of frequencies and range of impedances in which it was to operate. The transformer does __NOT__ have a bunch of "300 ohm" windings that can be connected in series or parallel. It has four identical windings that can be connected as desired. This gives one the following turns ratios: 1:1, 2:1, and 3:1. The impedance ratios vary as the square of the turns ratio. Therefore, if a transformer was fed from a 600 ohm source, the possible output impedances are 600, 150, and 66.6 ohms. To best distribute the internal reactances, when the 1:1 configuration is used, pairs of windings are series-connected. When 2:1 is used, the input pair is series-connected, and the output pair is connected in parallel. These are the connection!
 s that 
would be used in program channels. For mid frequencies, there would be no difference in impedance transformation for the 1:1 configuration if the windings were paralleled. However, the transformer would not have enough inductance to properly pass frequencies below about 100 Hz without distortion (the volts-per-turn would be too high). That's why the higher impedance side has the windings in series. The lower impedance side ends up with the same volts-per-turn because of the impedance transformation. To correctly distribute the leakage reactance (for high frequency response) the low impedance windings are paralleled.

To correct other misconceptions about wire transmission, one only  needs to look at the limits. Since an audio transmission line is a distribution of series inductance, series resistance, shunt capacitance, and some shunt (dielectric loss) resistance, we have relatively low Q delay line. If a delay line has identical elements, or is physically long enough so that its distribution "seems like" a bunch of identical elements, then the following occurs when you muck with its source impedance: If the source impedance is equal to its load impedance, and the delay line is matched, the line acts like a lowpass filter with a smooth (single pole) roll off at its high frequency limit. If its input is mismatched by feeding if from a lower impedance, the frequency response with contain a peak in amplitude just before its roll-off. Also, the roll-off with be faster, containing an additional pole that the mismatch creates. This characteristic can be used to "bring up" the high end when att!
 empting
 to equalize the line. If the line is fed from
an impedance that is higher than its designed impedance, there will be a high-frequency roll-off that occurs  at a lower frequency than the natural lowpass characteristics of the line. Playing with the load impedance does nearly the opposite, a lower impedance starts an early roll-off, and a higher impedance causes peaking.

In the limit of a zero source impedance, and an infinite terminating impedance, we have a resonant
circuit as f = 1 /  2pi * sqrt(LC)  where the L is the series inductance and the C is the shunt capacitance.
The wire-line has become a pi-network.

--
Cheers,
Richard B. Johnson
Read about my book
http://www.AbominableFirebug.com

 -------------- Original message ----------------------
From: "Dana  Puopolo" <dpuopolo at usa.net>
> You can use ONE winding to source 300 ohms. Put a 300 ohm resistor across the
> ubused winding.
> 
> -D
> 
> 
> ------ Original Message ------
> Received: Tue, 10 Apr 2007 09:05:25 AM EDT
> From: RichardBJohnson at comcast.net
> To: "Broadcasters' Mailing List" <broadcast at radiolists.net>,
> broadcast at radiolists.netCc: K7qa at aol.com
> Subject: Re: [BC] Xmitters at aol.com
> 
> 
> The impedance of various telephone cables is well documented. 16-gauge
> spiral-four toll entrance cable is 540 ohms -j460, 22 AWG emergency cable is
> 464 ohms -j449, paper insulated (inter-city toll cable) is 910 ohms for 26
> AWG, 22 AWG is 756 ohms, 19 AWG is 453 ohms, 16 AWG  is 320. All from Page
> 823, Wire Transmission, Reference Data for Radio Engineers, fourth edition.
> Nothing is close to 150 ohms. It is well known that the series 500 "standard
> black telephone" was called the "500" because its hybrid (anti sidetone
> network) was designed
> for 500 ohms. Historically, the telephone lines have been called "500 ohm"
> circuits.
> 
> Also, there are no 300-ohm windings on a 111-C. The transformer consists of
> four identical windings that are electrostatically shielded so they can be
> connected in any manner. The usual (read correct) connection is for two
> windings to be series-connected for the output, and two windings to be
> series-connected for the output.
> 
> See http://www.oldradio.com/current/111c.html , It shows the two usual methods
> of connection. Note that both the line-side and the drop side can be connected
> either in series or parallel. Don't be fooled by the "typical use" note. It's
> for "short" i.e., unequalized circuits.
> 
> I would guess that your response was either a test to see if I was still
> awake, or not very well thought out!
> 
> --
> Cheers,
> Richard B. Johnson
> Read about my book
> http://www.AbominableFirebug.com
> 
> 
>  -------------- Original message ----------------------
> From: K7qa at aol.com
> >  
> > In a message dated 4/9/2007 6:41:47 AM Pacific Daylight Time,  
> > RichardBJohnson at comcast.net writes:
> > 
> > The  circuit feeding the telco pair should not be connected as 150 ohms  
> > unless
> > there has been a problem obtaining the required frequency response.  
> > 
> > 
> > Most standard balanced telco multiconductor trunk lines used for many years 
> 
> > had an internal surge impedance of 135 to 150 ohms at audio frequencies. The
>  
> > 111C had individual 300 ohm windings for a good reason.  Connect the  111C 
> > transformer split windings in parallel to achieve 150 ohms on the line side 
> and 
> > in series for the 600 ohm equipment side. 
> >  
> > tm
> > 
> > 
> > 
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